Fabricating substrates with oxide layers is well known in the art. An example of which is US Publication No. 2009/00170287 to Yuta Endo et al. (the '787 application). The '787 application relates to a process for fabricating an SOI substrate that uses a cleave technique to transfer a layer. Many processing steps (bonding, heat treatment, lift-off, etching, oxidation) are carried out between successive anneals so that, as a result, it is not the same structure that undergoes each anneal in the '787 application. This document discloses localized oxidations at different moments in the fabrication process.
Another example of a known process for fabricating a multilayer structure is also described with reference to FIGS. 1A to 1F.
As shown in FIGS. 1A and 1B, a composite structure 100 is formed by assembling a first wafer 101 and a second wafer 102, for example made of silicon. The first and second wafers, 101 and 102, here have the same diameter. They could however have different diameters.
An additional oxide layer (not shown) may be formed on one or both wafers, 101 and 102, before placing them in contact, especially when fabricating an SOI (silicon-on-insulator) structure.
The first wafer 101 has a chamfered edge, namely an edge comprising a top chamfer 104 and a bottom chamfer 105. The role of these chamfers is to make handling the wafers easier and to prevent edge flaking that could occur if these edges were sharp, such flakes being a source of particulate contamination on the surfaces of the wafers.
In the example described herein, the wafer assembly is produced using the direct bonding (molecular adhesion) technique, a process that is well known to those of ordinary skill in the art.
Once the bonding is completed, the composite structure 100 undergoes a stabilizing anneal. The purpose this anneal is to strengthen the bond between the first wafer 101 and the second wafer 102, and to provide these two wafers with a protective oxide cover layer. For this purpose, the stabilizing anneal is carried out in an oxidizing atmosphere so as to form an oxide layer 110 on the entire exposed surface of the composite structure 100 (FIG. 1C). The oxide layer 110 thus forms a protective layer that protects the composite structure from chemical etching, especially during subsequent processing operations.
The structure 100 is then trimmed, which comprises essentially eliminating an annular portion of the layer 106 comprising the chamfer 105 (FIGS. 1D and 1E). Such trimming is necessary because the presence of the chamfer 105 prevents good contact from being achieved between the first and the second wafers at their periphery. There is therefore a peripheral region where the transferred layer is weakly bonded, or not at all bonded, to the second wafer. After transfer of the layer, this peripheral region of the transferred layer must be removed because it is liable to break in an uncontrolled way and to contaminate the structure with undesirable fragments or particles.
Preferably, a trimming process called hybrid trimming is used. This process includes carrying out a first trimming by an entirely mechanical action or by mechanical machining (FIG. 1D), followed by a second, at least partially non-mechanical trimming for trimming the remaining thickness of the first wafer (FIG. 1E). This second trimming corresponds in general to chemical trimming that is selective with respect to the oxide layers formed on the wafer(s) 101 and/or 102. The hybrid trimming especially prevents peel-off both at the bonding interface between the transferred layer and the second wafer and in the transferred layer itself.
In the example described here, the chemical trimming comprises chemical etching or wet etching. The chemical etching solution is chosen depending on the material to be etched. In this example, for etching silicon, either of tetramethyl ammonium hydroxide (TMAH) or potassium hydroxide KOH solutions can be used, with both solutions being well known to those skilled in the art. The first wafer 101 may then be thinned so as to form a transferred layer 106 having a predetermined thickness e (FIG. 1F). This thickness e may for example be about 10 μm.
Oxidation of the composite structure 100 (FIG. 1C) before the trimming operation prevents the second wafer 102 from being chemically etched by a TMAH solution during the chemical trimming. The oxide layer 110 also protects the surface of the first wafer 101 from the trimming chemical etch, before it is thinned.
After chemical trimming, however, the appearance of defects has been observed at the edges of the composite structures. More specifically, the defects observed are nicks distributed over the edges (or edge face) of the second wafer of the composite structures. These nicks are undesirable for the fabricator since they may, for example, be the source of flakes coming from the second wafer, these flakes being liable to contaminate the exposed surface of the first wafer. More generally, these defects attest to a non-optimized fabrication process and as a result, the multilayer structures thus produced are less attractive. These defects may moreover cause various problems during supplementary technological steps on these defective composite structures, such as during the fabrication of microcomponents on the exposed surface of the first wafer.
It is therefore necessary to produce composite structures without such defects even when the process for fabricating these structures comprises one or more steps implementing chemical etches. Such chemical etches may occur especially, but not exclusively, during a trimming or thinning operation on the first wafer. The present invention now provides such structures.